1. Field of the Invention
The present invention relates to a microwave transponder for automatic identification systems that have a back scatter function.
2. Related Art
A back scatter system can be defined as a system in which the data carrier, the so-called transponder, includes antennas and modulation circuits. These components modulate information sidebands with the data to be read from a microwave signal falling on the transponder from a reading unit, thus generating a reflected signal containing information for reception and decoding by the reading unit, without supplying fresh energy. Certain embodiments also enable data to be written into the transponder by modulating the amplitude of the signal from the reading unit with detection data, and memory programming in the transponder. This technique is well known and will not therefore be described in detail here.
One problem with microwave-based transponders of this kind, i.e. transponders that operate at 0.9 GHz and even higher frequencies, is that they are normally unable to communicate omnidirectionally, due to the directivity of the antenna system embodied in the transponder. This makes it necessary to orientate the transponder so that a given side of the transponder will face towards the reading unit, which is difficult to achieve when, e.g., the receiving unit is concealed in a package, a handbag, pocket or like receptacle.
Although the earth plane can be reduced to approximately the same size as the antenna plane with the aid of a patch antenna, so as to obtain equal sensitivity in both forward and rearward directions, transponder readings may become sensitive to reflections from a rearwardly lying reflection plane, should this plane be located at a distance that causes leakage of the information carrying signal from the transponder antenna through the signal reflected via the reflection plane. Packaged objects contained in handbags and the like are often reflective and cause uncertain readings. The same applies to the human body, when the transponder is carried in a pocket.
The necessity of taking into consideration the orientation of the data carrier and its proximity to surrounding objects in order to obtain positive readings is a troublesome limitation in each of these cases. Transponders equipped with dipolar antennas are subjected to similar problems, since these transponders are also influenced by the possible presence of a reflection plane in the proximity of the antenna.
Another drawback with antennas of this type is their low antenna gain, i.e. only a minor part of the incident signal is reflected back to the reading unit, due to lack of directivity, which shortens the range of the system.